There are various situations where it is important to detect the presence of specific gases in an atmosphere. Certain gases may be harmful to humans making it desirable to monitor a system's environment to ensure that the concentration of selected gases does not exceed certain threshold limits. Pressurized systems also need to be monitored for leaks to ensure they are functioning properly to avoid future damage. Leak detection, however, can be a complex and costly endeavor. Depending on the system and the application where a gas or liquid may be stored, the sensitivity of a leak detector to a given substance is a complex function of operational, environmental, health and economic issues.
For example, air escaping out of a compressor tank or air line may be difficult to locate and repair because the sound it makes can be masked by other sounds or the location may be invisible or inaccessible. Other examples are the leakage from air conditioning equipment, fire extinguishing equipment, or refrigerant gas out of a refrigeration system. Where a refrigeration system is concerned, for example, the functional, environmental, health and economic issues are unique. Government regulations may prohibit the leakage of a refrigerant gas above a certain level. Loss of refrigerant may be accompanied by loss of lubricant, which will affect the function of the system. Both of these conditions alone will generate cost to the owner, which will have to act within constraints of the law and economic capabilities to properly maintain the system.
Depending on the nature of the system, locating the various types of possible leaks may require completely different tools and methodologies. Location of a refrigerant leak may require a panoply of tools and equipment since, for a given situation, there may be a number of refrigerant gases each requiring a special detector. The gas families used in refrigeration allow the use of sensors that “cross-over”, meaning that a particular sensor optimized to work best for one gas, such as R12, will also work for another gas, such as R134A. However, if the gas is from a different family, such as R422, the sensor used in the detection of R12 gas will not be as sensitive. Sensitivity of sensors for a particular gas or family of gases is referred to as the minimum detectable amount (MDA) and is measured in parts per million (ppm).
Reliability of readings from gas sensors, however, can be misleading since the dispersion of leaking gas in air results in the density of the gas varying according to the distance the measurement is taken from the leak source. For example, suppose the ultimate sensitivity of a leak detector is 10 ppm for a given type of gas. If the dilution of the gas in air is such that there is only 1 ppm, the detector will not detect its presence. Such a situation is possible when there is wind blowing the leaking gas, thereby dispersing it, and in effect diluting it. Situations such as this make it very difficult to ascertain the existence of a leak and pinpoint its location because, to trace the gas to the leak, the sensor must collect enough gas and the density of this gas must stay within the sensor's capabilities. Even though the leak rate may be orders of magnitude over the MDA of the sensor, a wind's dispersion effect may reduce it to below the MDA. Such a situation can be quite common in refrigeration and A/C field servicing. Accordingly, a technician needs to carry several leak detectors since they compliment each other in the quest of locating a leak.
Within the family of gas sensors, also referred to as gas detectors, are the chemical properties leak detectors (CPLDs). CPLDs are very sensitive and can reach an MDL of 0.1 oz per year, but suffer from contamination, wind dilution and saturation. CPLDs are based on ionization or ion capture of the leaking gas. Special sensing elements are used to generate a signal when the gas is present. Examples of CPLDs are discussed in the following patents: U.S. Pat. No. 5,104,513 to Lee et al., U.S. Pat. No. 5,932,176 to Yannopoulos et al., U.S. Pat. No. 3,991,360 to Orth et al., and U.S. Pat. No. 4,045,729 to Loh.
Another type of gas sensor, known as the thermal conductivity detector (TCD), compares the thermal conductivity of air to a gas that is drawn by heating a wire or thermal sensor. Changes to the thermal balance of the wire causes the sensor to detect the presence of a gas. Sensors using thermal conductivity, while suffering from the same problems as the CPLD type sensors, can detect inert gases at low levels that are undetectable by CPLDs and ultrasonic sensors. An example of a commercially available leak detection instrument which utilizes a TCD is the LeakCheck, sold by EFD Instruments of NY. Gas sensors can also be of a variety of other types including the Photo Ionization type (PID), such as discussed in U.S. Pat. Nos. 5,561,344 and 6,509,562, the chemical detector type (CD), the laser interferometer type (LID), the corona discharge type (CDD), microelectromechanical systems (MEMS) based sensors, or surface acoustic wave (SAW) sensors, to name a few.
Other types of known detectors can broadly be characterized as listening devices because they listen to the sound caused by leak flow into or out of a system. This sound can be either air-borne or structure-borne and be in the sonic or ultrasonic range. Listening devices of this type generally utilize an acoustic emissions (AE) sensor to detect the leak. One particular type of listening device is known as an ultrasonic leak detector (ULD). There are a number of ULD instruments available, such as those described in my following patents: U.S. Pat. No. 5,103,675, U.S. Pat. No. 5,432,755, U.S. Pat. No. 5,436,556, U.S. Pat. No. 6,058,076, U.S. Pat. No. 6,079,275, and U.S. Pat. No. 6,163,504. Each of my earlier ULDs employs an AE sensor, either alone or in conjunction with a touch probe, to conveniently detect air-borne sound, structure-borne sound, or both.
ULDs are very useful in refrigeration systems since they can detect vacuum leaks and are not affected by wind. ULDs listen to the sound the flow of a leaking gas makes as it escapes from a container or is being sucked in under vacuum. Sound is generated as the gas expands and its flow becomes turbulent. Because of this principle, ULDs can detect any type of gas. Under ideal conditions, the minimum flow ULDs can detect is approximately 0.01 SCCM (standard cubic centimeters per minute). Their ultimate sensitivity, though, does not reach the desired leak flow rate of 0.5 oz per year in the refrigeration field. Additionally, background noise can make it difficult to locate the leak point. Thus, leak detection with ULDs can also have its limitations.
Another approach to ascertaining the presence of gases, for example refrigerant gases which have been injected with a dye, is through the use of ultraviolet (UV) illumination. This causes the gas, or its residue, to fluoresce, thereby leaving a visual indication of its presence.
While the art is ripe with numerous approaches for detecting leak characteristics, these various techniques have essentially evolved in isolation. The result has been that service technicians often need numerous tools at their disposal to effectively monitor leaks. This can become cumbersome and often results in inefficiency, inconvenience, and added cost. Accordingly, there is a need to overcome these disadvantages so that technicians servicing any type of appliance that is charged, for example with a refrigerant gas, can do so reliably, in a time-efficient manner and with fewer tools. The present invention is directed to meeting these needs.